Femoral head-neck prosthesis and method of implantation

ABSTRACT

A femoral head-neck prosthesis ( 1 ) which allows natural straining of the upper femur to prevent bone loss. The natural angle of loading of the bone is determined prior to the operation, and the prosthesis is implanted with its longitudinal axis (AX- 1 ) parallel to the natural angle (AX- 5 ). The prosthesis is constructed to inhibit axial fixation and “splinting” of the prosthesis below the interface between the femur neck and the prosthesis on the upper femur. Splines ( 19 ) on a stem ( 13 ) of the prosthesis help to fix the prosthesis against rotation and toggling motion. The prosthesis is asymmetrical about its longitudinal axis to provide further stability when implanted. The prosthesis and its method of implantation preserve the trochanter and cap the femur to prevent microscopic debris from entering the interior bone.

This application is a continuation-in-part of U.S. application Ser. No.60/023,398 filed Aug. 13, 1996 which is incorporated herein byreference.

BACKGROUND OF THE INVENTION

This invention relates generally to femoral head-neck prostheses andmethods for their implantation.

Total hip replacement became a clinical reality for the first time inNovember, 1963, because of “cement fixation” of the components. Thefemoral head and neck were removed, the upper marrow canal of the femurwas cleaned out (i.e., marrow contents removed), acrylic cement waspoured into the marrow canal of the femur, and the metal femoralcomponent was inserted into the liquid cement. In 10 to 15 minutes, theacrylic (methylmethacrylate) cement hardened and provided fixation forthe femoral stem. The acrylic cement is similar to the acrylic dentistsuse to make dentures. Today, cement fixation of femoral components isstill the most common means of fixation of the implant to the bone.

Cement fixation is the ultimate in form-filling contact with the bone.The liquid cement touches the entire inner surface of the upper femur.This type of fixation is generally successful for the short term (tenyears), however in the long run, deterioration of the bone occurs andthe cement and femoral component may loosen. Bone loss is caused by thecement and implant splinting the upper femur, preventing the upper femurfrom being subjected to natural bending. This is particularly a problemwith younger patients (i.e., less than 50 years old).

Non-cemented or “press-fit” femoral components basically try to do thesame thing as cemented implants; achieve solid fixation between theimplant and the bone by maximally filling the medullary canal with themetal implant. In other words, the thinking is that the more closely andcompletely the metal implant fills the medullary canal, the better thefixation will be, and the more successful the result will be. However,experience shows this is not always the case.

The non-cemented femoral stems are larger and thicker than theircemented counterparts because the more flexible layer of acrylic cementis replaced with metal. Because a non-cemented stem is made of the samematerial and has a greater diameter than the cemented stem, it isstiffer. The greater the stiffness, the worse the splinting of the upperfemur from the normal bending deflection that occurs in walking(strain). Although acceptable clinical results are achieved withnon-cemented intramedullary femoral stems, non-cemented stems enerallyhave a more rapid rate of bone loss in the upper femur due to straindeprivation or what is commonly but incorrectly referred to as “stressshielding.”

In summary, the fixation of all conventional intramedullary total hipfemoral components depends on maximally filling the upper femur and themedullary canal with either cement and metal or metal alone. Notcoincidentally, bone loss occurs with all of these implants.

SUMMARY OF THE INVENTION

Among the several objects and features of the present invention may benoted the provision of a femoral head-neck prosthesis which protects thefemur from bone loss; the provision of such a prosthesis which providesstable seat between the prosthesis and the neck of the femur; theprovision of such a prosthesis which accommodates compression at itsinterface with the upper femur; the provision of such a prosthesis whichinhibits splinting of the upper femur; the provision of such aprosthesis which inhibits total axial fixation of the prosthesis belowthe seat; the provision of such a prosthesis which receives loads fromthe hip almost completely in compression; and the provision of such aprosthesis which has a longer useful life.

Further among the several objects and features of the present inventionmay be noted the provision of a method for implanting a femoralhead-neck prosthesis which considers the historical loading of thefemur; the provision of such a method which results in loads applied tothe prosthesis being transmitted in a substantially natural way to thefemur; the provision of such a method which permits a stable interfacebetween a collar of the prosthesis and the femur neck for transmissionof loads to the neck and upper femur; the provision of such a methodwhich inhibits total axial fixation of the prosthesis; the provision ofsuch a method which substantially reduces bending moments on theprosthesis as implanted; and the provision of such a method which causesthe prosthesis to be loaded almost completely in compression.

Generally, a femoral prosthesis for implantation in a femur comprises aneck adapted to receive a prosthetic head thereon, a collar on which theneck is mounted and a stem extending from the collar on the oppositeside of the collar from the neck. The prosthesis has a longitudinal axiscorresponding to the longitudinal axis of the stem. The stem isconstructed and arranged to fix the prosthesis from movement about itslongitudinal axis and about axes perpendicular to the longitudinal axis,and to inhibit axial fixation of the prosthesis upon implantation in thefemur, thereby to achieve substantially natural loading of the upperfemur.

Another aspect of the present invention is a method for implanting anon-cemented femoral head-neck prosthesis in a femur, the femur having ashaft and a neck at the upper end of the shaft at the medial side of thefemur. Generally, the method includes the steps of determining the axisof the medial trabecular stream of the femur, and cutting the neck ofthe femur to form a seat on the femur neck. A first bore is drilledthrough the shaft of the femur to extend from the neck of the femur downtoward the lateral side of the femur along a line substantially parallelto the axis of the medial trabecular stream. A second bore is drilledthrough the shaft of the femur to extend from the neck of the femur downtoward the lateral side of the femur along a line substantially parallelto the axis of the medial trabecular stream but spaced from the line ofthe first bore. A stem of the prosthesis is inserted in one of the firstand second bores extending through the shaft to the lateral side of thefemur, with a portion of the stem being received in the other of thefirst and second bores.

A fundamental aspect of this device is the stem being implanted in linewith the normal loading trajectory of each individual hip in accordancewith my prior U.S. Pat. No. 4,998,937. With the stem implanted in thisorientation, the main forces on the implant will be end-on (i.e., incompression). In other words, with each step, the 500 pounds of forcethat a 150 pound man generates at the hip with normal walking isdirected along the axis of the implant. The goal is to have the femoralneck receive 100% of the load (joint reaction force) through the collar.Force on the ball of the implant forces the collar against the resectedfemoral neck. The goal is to have the collar transmit all the load tothe femoral neck so that the bone will receive 100% of the normal strain(bending).

Because so much load is transmitted through the collar, it is alsoimportant that the collar-bone interface be stable. In my previouspatented (stem/barrel/plate) design, the barrel/sideplate componentstabilized the stem and collar. The barrel prevented the stem fromtoggling (forces which move the ball front to back or side to side) orrotating. The collar/stem component was free to be dynamicallycompressed against the femoral neck because the stem in the barreloffered little or no resistance to axial movement relative to the femur.

In my new invention, the upper femur takes the place of thebarrel/sideplate in the prevention of toggling and rotation of theimplant. The implant is constructed to mate with the machined upperfemur so that toggle and rotation are controlled, but compression ispermitted. The thick proximal stem makes contact with the inside of thefemoral neck to prevent toggle and rotation. After cutting the femoralneck and removing all of the marrow contents, if you sight along theaxis of loading, you will see a cavity in the femur which is generallyoval in cross section. This oval cross-section can be substantiallyfilled with two overlapping circles, one smaller than the other. Theoverlapping circles, extended along the axis of loading into threedimensions become two overlapping cylinders. The stem of my new femoralprosthesis has a double cylinder geometry. The upper femur cavity ismachined to allow this double cylinder geometry to fit in the upperfemur.

The geometry of the inside of the upper femur is variable from person toperson. The goal is to have the surface of the double cylinder shapedupper stem of the prosthesis contact the bone of the femur in at leastone point at all locations along the length of the upper stem. Becauseof the irregular shape of the bone, and because of the constraint ofcylindrical reaming parallel to one axis (i.e., the load axis),implant-bone contact along the upper stem will be incomplete. Thestraight sides of the implant will contact the curving surfaces of theupper femur in only some discrete areas. This tangential contact isbelieved to be adequate to control toggle and rotational motion yetallow the bone to be compressed.

The machining of the upper femur basically comprises four basicdrilling, reaming and circular planing operations.

Step 1. The medial femoral neck cavity is reamed on a first axis to fitthe smaller of the two cylinders.

Step 2. A hole is drilled in femur on the same first axis.

Step 3. The lateral femoral neck cavity is reamed on a second axisparallel to the first to fit the larger of the two cross-sectionalcircles.

Step 4. The neck is “planed” to match the shape of the collar.

The implant is then inserted into the femur, with portions received inboth of the cross-sectional circles.

Other objects and features of the present invention will be in partapparent and in part pointed out hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a fragmentary cross section of an upper femur showing afemoral head-neck prosthesis of the present invention implanted in thefemur (the prosthesis being shown in full lines);

FIG. 1A is a view of an intact femur showing the medial trabecularstream of the femur and axes of the femur and prosthesis;

FIG. 1B is a cross-sectional view through the femoral neck illustratingthe planes of the femur;

FIGS. 2A-2D illustrate areas of contact between he upper prosthesis andbone at locations indicated by lines 2A—2A through 2D—2D, respectively;

FIG. 3 is a cross-section through the splined portion of the lower stemtaken in the plane of line 3—3 of FIG. 1;

FIGS. 4A-R are schematic views of a lesser preferred embodiment and apreferred embodiment of a method of implanting the prosthesis:

FIG. 4A is a view of the femur showing the axis of the medial trabecularstream;

FIG. 4B is a view showing the angle guide and the saw guide around thefemur for femoral neck resection;

FIG. 4C is a view showing the angle guide, a reamer guide and a reamerfor reaming of the second bore in the femoral neck;

FIG. 4D is a view showing the reamer for reaming the second bore in thefemoral neck;

FIG. 4E is a view showing the angle guide, calcar milling guide andcalcar miller for milling the first bore in the femoral neck;

FIG. 4F is a view showing the calcar milling guide and calcar miller formilling the first bore in the femoral neck with the angle guide omittedfor clarity;;

FIG. 4G is a view showing a drill pin guide, a trocar point guide pinand a drill point guide pin for drilling through the lateral femoralcortex;

FIG. 4H is a view showing the drill point guide pin and a cannulatedcortex drill for drilling through the lateral femoral cortex;

FIG. 4I is a view showing the insertion of a calcar planing guide in thefemoral neck;

FIG. 4J is the view of FIG. 4I, but with the calcar planing guideremoved;

FIG. 4K is a view showing the calcar planing guide and the calcar planerfor planing of the femoral neck;

FIG. 4L is a view showing the implantation of the prosthesis;

FIG. 4M shows the start of the more preferred implantation steps and isa view showing the calcar miller for milling the first bore;

FIG. 4N is a view showing the angle guide, calcar milling guide,cannulated pin guide, trocar point guide pin and drill point guide pinfor drilling through the posterolateral femoral cortex;

FIG. 4P is a view showing the drill point guide pin and cannulatedcortex drill for drilling through the posterolateral femoral cortex;

FIG. 4Q is a view showing the offset reaming guide and cannulated reamerfor reaming the second bore in the femoral neck;

FIG. 4R is a view showing the calcar planing guide and calcar planer forplaning of the femoral neck;

FIG. 4S is a view showing the implanted prosthesis;

FIG. 5A is a perspective view of the split stem prosthesis of FIG. 1;

FIG. 5B is a front elevational view thereof;

FIG. 5C is a left side elevational view thereof;

FIG. 5D is a right side elevational view thereof;

FIG. 5E is a top plan view thereof;

FIG. 5F is a bottom plan view of the split stem prosthesis;

FIG. 5G is a sectional view of the split stem prosthesis taken in theplane of line 5G—5G in FIG. 5E;

FIG. 5H is an enlarged bottom end view of the stem of the split stemprosthesis showing splines on the stem;

FIG. 6 is a perspective view of a solid stem prosthesis;

FIG. 7A is a perspective view of an angle guide;

FIG. 7B is a left side elevational view thereof;

FIG. 7C is a front elevational view thereof;

FIG. 8A is a perspective view of a bracket of an angle guide;

FIG. 8B is a front elevational view thereof;

FIG. 8C is a top plan view of a bracket thereof;

FIG. 8D is a left side elevational view thereof;

FIG. 9A is a perspective view of an arm of the angle guide;

FIG. 9B is a front elevational view thereof;

FIG. 9C is an enlarged, fragmentary front elevational view of the leftend of the arm;

FIG. 9D is a top plan view of the arm of the angle guide;

FIG. 9E is a left side elevational view thereof;

FIG. 10A is perspective view of a calcar miller guide;

FIG. 10B is a front elevational view thereof;

FIG. 10C is a front elevational view thereof;

FIG. 10D is a bottom plan view thereof;

FIG. 11A is a perspective view of a calcar miller;

FIG. 11B is an elevational view of the calcar miller;

FIG. 11C is a bottom end view of the calcar miller;

FIG. 12A is an elevational view of a cannulated pin guide;

FIG. 12B is a perspective view of the cannulated pin guide;

FIG. 12C is a bottom end view thereof;

FIG. 12D is a fragmentary, elevational view of the cannulated pin guideshowing the bottom end;

FIG. 13A is a perspective view of the cannulated cortex drill;

FIG. 13B is a front elevational view of a cannulated cortex drill;

FIG. 13C is a bottom end view thereof;

FIG. 13D is a fragmentary front elevational view of a top end of thecannulated cortex drill;

FIG. 14A is a front view of the offset reaming guide;

FIG. 14B is a front elevational view thereof;

FIG. 14C is a left side elevational view thereof;

FIG. 14D is a top plan view of the offset reaming guide;

FIG. 14E is a bottom plan view of the offset reaming guide;

FIG. 15A is a perspective view of the cannulated reamer;

FIG. 15B is an elevational view of a cannulated reamer;

FIG. 15C is a bottom end view of the cannulated reamer;

FIG. 15D is an enlarged, fragmentary elevational view of the cannulatedreamer showing an upper end;

FIG. 16A is a perspective view of a calcar planing guide;

FIG. 16B is a left side elevational view thereof;

FIG. 16C is a top plan view of the calcar planing guide;

FIG. 16D is a front elevational view of a calcar planing guide of asecond embodiment;

FIG. 17A is a perspective view thereof;

FIG. 17B is a front elevational view of a calcar planer;

FIG. 17C is a bottom end view thereof;

FIGS. 18A-18N and 18P illustrate a most preferred embodiment of a methodfor implanting the prosthesis;

FIG. 18A is a view of the femur showing the axis of the medialtrabecular stream;

FIG. 18B is a view showing setting of the angle guide prior to mountingon the femur;

FIG. 18C is a view showing the angle guide and the saw guide around thefemur for femoral neck resection;

FIG. 18D is a view showing an initial reaming step;

FIG. 18E is the view of FIG. 18D in vertical section with the angleguide removed;

FIG. 18F is a view showing sizing the femur for selection of theappropriate prosthesis;

FIG. 18G is a view showing a calcar miller milling the first bore;

FIG. 18H is a view showing a pin guide and drill point guide pin for usein drilling through the posterolateral femoral cortex;

FIG. 18I is a view showing the drill point guide pin after removal ofthe pin guide;

FIG. 18J shows a cortical drill and sleeve used to drill theposterolateral femoral cortex;

FIG. 18K shows the cortical drill as received on the guide pin afterdrilling through the posterolateral femoral cortex;

FIG. 18L is a view showing an offset reaming guide and cannulated reamerfor reaming the second bore in the femoral neck;

FIG. 18M is a view showing the calcar planing guide and calcar planerfor planing of the femoral neck;

FIG. 18N is a view illustrating installation of the prosthesis employinga removable bullet tip;

FIG. 18P is a view illustrating an installed prosthesis;

FIG. 19A is a side elevation of a prosthesis having and a saw templatemounted on the prosthesis for use in seating the prosthesis;

FIG. 19B is a fragmentary cross section of an upper portion of the femurand saw template as shown in FIG. 19A, as seen from a position generallymedial of the femur;

FIG. 19C is a top plan view of the femur and saw template;

FIG. 19D is a section taken in a plane including line 19D—19D of FIG.19A;

FIG. 20 is a plot of test results on the prosthesis of the presentinvention.

Corresponding reference characters indicate corresponding partsthroughout the several views of the drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

(a) The Femoral Head-Neck Prosthesis

Referring now to the drawings, and in particular to FIGS. 1, 1A and 1B,a transosseous, non-cemented femoral head-neck prosthesis of the presentinvention (indicated generally at 1) is shown as implanted in a femur F.The femur includes a femoral shaft S, a femoral head H, neck N and agreater trochanter T at the upper end of the shaft at the lateral sideof the femur. The femur F has a hard layer of cortical bone C adjacentthe surface of the bone, relatively soft cancellous bone and endosteum(not shown) inside the femur. The prosthesis 1 is made of cobalt-chromealloy, titanium or other suitable material, and has a longitudinal axisgenerally indicated at AX-1. As implanted, the prosthesis 1 extendsgenerally from the resected femoral neck N diagonally across themedullary canal MC and out (posterolaterally) an opposite side of thefemur. The prosthesis 1 is of the type which is not cemented into thefemur F, but is secured by mechanical interconnection of the prosthesiswith the bone, as described more fully hereinafter. The prosthesis 1 isconstructed so that it is securely held in the bone from rotation (aboutits longitudinal axis AX-1) and toggling (anterior-posterior andmedial-lateral) motion, while permitting axial micromotion to achievenatural bone loading condition thereby to preserve the bone.

The prosthesis 1 has a generally spherical ball 3 which is received in acup (not shown) implanted in the hip socket (not shown) to permitmovement at the hip joint. Referring now additionally to FIGS. 5A-5H,the ball 3 is fixedly attached to an upper portion 5A of a neck 5 of theprosthesis which is received in a hole (not shown) in the underside ofthe ball. The neck 5 is generally cylindrical in shape and includes alower portion 5B below the upper portion 5A which is of a smallerdiameter than the upper portion. The lower portion SB of the neck ismounted on a collar (generally indicated at 7) of the prosthesis 1 whichrests against the femoral neck N, as shown in FIG. 1, and transmitsloads to the upper femur. As shown, the collar 7 is continuous about thecircumference of the prosthesis 1 and extends outward laterally,anteriorly, medially and posteriorly to cap the medullary canal MC. Asshown, the collar 7 is continuous about the circumference of theprosthesis 1 and extends outward laterally, anteriorly, medially andposteriorly to cap the medullary canal MC.

The collar 7 includes a neck platform 9 on which the neck 5 is mounted,and a curved flange 11 which engages the greater trochanter T of thefemur. The underside of the neck platform 9A has a slight frustoconicalshape and the underside of the flange 11A has the shape of a section ofcone. In the preferred embodiment, the underside 9A of the platformmakes an angle of about 10° with a plane perpendicular to thelongitudinal axis AX-1 of the prosthesis. The shape of the underside 9Aand its close correspondence to the shape of the seat formed on theresected neck N allow the collar 7 to cap the medullary canal MC andinhibit migration of debris into the medullary canal after implantationof the prosthesis 1. The curved underside 11A of the flange makes anangle of about 60° with the same plane. Thus, the underside (9A, 11A) ofthe collar 7 defines a compound angle. The flatter neck platform 9 lieson the partially resected femoral neck N, and the flange 11 restsagainst the greater trochanter T of the femur. The greater trochanter isthe primary sight of muscle attachment to the femur F at the hip. Theupstanding flange 11 permits the collar 7 to solidly support theprosthesis 1 on cortical bone C on the upper femur while allowing mostof the greater trochanter T to be preserved. Use of a substantially flatcollar (not shown) would require resection of a substantial portion ofthe trochanter T to provide room for the collar. The underside (9A, 11A)of the collar 7 of the present invention engages and is supported by thehard cortical bone C of the femur.

In the preferred embodiment, the underside 9A of the neck platform 9 andunderside 11A of the flange 11 are coated with a porous material (notshown) to facilitate bone growth into the collar 7 where it rests on theupper end of the femur F. However, the remaining portions of the collar7 and all other parts the prosthesis 1 preferably remain free of porouscoating, roughening or other construction which would encourage bonegrowth into the prosthesis. It is to be understood that the use ofporous coating or other structure to facilitate bone ingrowth into theprosthesis 1 may be other than described and still fall within the scopeof the present invention.

A stem, generally indicated at 13, mounted on the underside of thecollar 7 extends generally downwardly through the femur F. In thepreferred embodiment, the neck 5, collar 7 and stem 13 are formed as onepiece. The longitudinal axis AX-2 of the neck 5 is parallel to thelongitudinal axis of the stem, which is coincident with the longitudinalaxis AX-1 of the prosthesis 1. As installed, the prosthesis 1 issubstantially parallel to an axis AX-5 (FIG. 1A) corresponding to thedirection of the normal loading vector of the hip so that forces fromthe hip are applied compressively to the neck 5 which transmits thoseforces (via the collar 7) compressively to the femoral neck N. Axialfixation of the prosthesis 1 in the bone is achieved by bone ingrowth ofthe upper femur F into the collar 7. As described more fullyhereinafter, axial fixation of the stem 13, caused by bone ingrowth intothe stem and/or strain hardening of bone engaging the stem, is preventedby construction of the stem.

The stem 13 includes an upper portion and a lower portion (designatedgenerally by reference numerals 15 and 17, respectively). The radiallyoutwardly facing surfaces of the stem 13 disposed for engaging theinterior of the femur F are, broadly, “fixation surfaces.” The lowerportion 17 is sized for a close fit within the femur F, and haslongitudinally extending splines 19 (see FIGS. 3 and 5H) which penetratethe bone inside the femur to secure the prosthesis 1 in the femur. Thelower portion 17 has a longitudinal split 21 to accommodate normal loaddeflection of the proximal femur. The splines 19 hold the prosthesis 1securely against rotational movement about the longitudinal axis AX-1 ofthe prosthesis after implantation, and encourage bone growth between thesplines. However, although the splines 19 resist axial displacement ofthe prosthesis 1 relative to the femur F, the splines do not rigidly fixthe prosthesis against axial micromotion. To provide additional fixationof the prosthesis 1, splines (not shown) may also be formed on the upperportion 15 of the stem.

A more preferred embodiment of a prosthesis 1′ is shown in FIG. 6 has asolid lower stem portion 17′. It is believed that the solid stemprovides for greater accuracy in installation and prevents axialfixation which potentially might occur through ingrowth of bone into theslot 21 of the prosthesis 1. A still more preferred embodiment of aprosthesis 1″ is shown in FIGS. 18P and 19 to have a flat underside 9A″of the collar 7″.

The distal end of the lower portion 17 of the stem 13 is cut on an angleto the longitudinal axis, so that the distal end of the lower portion issomewhat pointed. Moreover, the distal end of the lower portion 17 isgenerally aligned with or parallel to the outer surface of the femur Fon the posterolateral side. The lower portion 17 preferably extendsoutwardly from the posterolateral side of the femur F to inhibit bonegrowth over the distal end of the lower portion which would fix theprosthesis 1 in an axial direction and prevent the natural loading atthe upper end of the femur by the collar 7.

The upper portion 15 of the stem 13 generally has the shape ofoverlapping cylinders near the collar 7 (see FIG. 5G). A firstoverlapping cylindrical element of the upper portion is designated 23,and a second overlapping cylindrical element of the upper portion isdesignated 25. The first (smaller) cylindrical element 23 is co-axialwith the longitudinal axis AX-1 of the prosthesis 1, while the second(larger) cylindrical element 25 has an axis which is parallel to thefirst cylindrical element and radially offset a distance from the axisAX-1 less than the sum of the radii of the first and second cylindricalelements. The first cylindrical element 23 has a diameter greater thanthe coaxial stem lower portion 17 of the stem. The diameter of the lowerportion 17 is kept small to minimize the size of the opening formed inthe posterolateral femoral cortex. As an example, if the diameter of thefirst element 23 were 15 mm, the diameter of the lower portion 17 wouldbe about 12 mm. The shape of the upper portion 15 is defined by theportions of the first and second cylindrical elements 23, 25 which arenot overlapping. The offset, eccentric location of the second element 25causes the upper portion 15, as received in the bores 3B, B2 to hold theprosthesis against rotation about axis AX-1. A lower end surface 25A ofthe second cylindrical element is cut in a plane which makes an angle ofapproximately 30° with respect to the longitudinal axis AX-1.

As illustrated by FIGS. 2A-D, the upper portion 15 of the stem 13contacts the endosteal neck cortex of the femur F only in discrete areasaround the circumference of the upper portion. The cross sectional viewsof the drawings (taken as indicated in FIG. 1) schematically illustratethe regions of engagement of the cortex and the upper portion 15 of thestem at four distinct locations along the length of the upper portion.It will be noted that engagement does occur at three spaced apartlocations around the upper portion 15 so that the upper portion is ableto provide good fixation against both rotation motion of the prosthesis1 about its longitudinal axis AX-1 and toggling motion of the prosthesisabout axes perpendicular to the longitudinal axis.

However, the discrete areas of contact do not rigidly fix the upperportion 15 of the stem 13 against axial movement relative to the femurF. The limited area of contact reduces the frictional interaction of theprosthesis 1 and bone in the endosteal neck cortex. Moreover, the upperstem portion 15 has smooth exterior walls which substantially preventbone from growing into the upper stem portion thereby to prevent axialfixation of the prosthesis by bone ingrowth. Thus, the upper stemportion 15 will not prevent loads from the hip from being appliedcompressively to the upper end of the femur F. This more natural loadingof the femur induces more natural straining of the upper femur andprevents deterioration of the upper femur, which is important tomaximizing the useful life of the implanted prosthesis 1.

(b) Instruments used to Implant the Prosthesis

While a number of different instruments may be helpful for implantingthe femoral head-neck prosthesis 1, an angle guide generally designatedat 29 and shown in FIGS. 7A-7C is particularly adapted to be removablysecured to the femoral shaft S for holding a plurality of cutting,drilling and reaming accessories in position with respect to the femurF. The angle guide 29 comprises a bracket 31 (see FIGS. 8A-8D) having afirst member 31A adapted to be removably secured by a suitable clamp(not shown) in face-to-face engagement with the femoral shaft S. Asecond member 31B extends outwardly from the first member 31A andincludes an arcuate faceplate 31C. A guide sleeve 33 or outriggerportion (see FIGS. 9A-9E) is capable of extending at a selected angleupwardly and outwardly from the bracket 31 at one side of the femoralshaft. The guide sleeve 33 includes a mounting member 33A attached tothe bracket 31 by a screw 35. The guide sleeve 33 may be angularlyadjusted relative to the bracket 31 by loosening the screw 35 andturning the guide sleeve on the screw to a selected angular position.The faceplate 31C carries indicia which are pointed to by a pointer 37associated with the mounting member 33A to show the angle of the guidesleeve. The angle is selected so that the guide sleeve 33 extends fromthe bracket 31 along a line substantially parallel to the previouslydetermined average compression loading vector (the “normal” direction inwhich the femur F is loaded, AX-5) for the femur of the specific patientwhen the bracket is attached to the Femur. The guide sleeve 33 has athrough hole 39 for receiving and holding other instruments in the sameangular position as the guide sleeve.

As noted above, angle guide 29 is adapted for holding a variety ofdifferent instruments used in implanting the prosthesis 1 of the presentinvention. One such instrument is a saw guide 41 (see FIG. 4B) which canbe detachably mounted in the through hole 39 of the guide sleeve 33 forguiding a saw blade (not shown) to cut the femoral neck N. The saw guide41 has a sawcut slot 41A generally perpendicular to the centrallongitudinal axis of guide sleeve 33. The saw guide 41 is slidablyadjustable in the through hole 39 to properly position it with respectto the femoral neck N. A set screw 43 of the angle guide 29 is providedfor securing the saw guide 41 in adjusted position.

Referring now to FIGS. 10A-11D a calcar miller guide, generallyindicated at 45, has an outrigger portion 47 receivable in the throughhole 39 of the angle guide 29 for mounting on the angle guide in thesame manner as the saw guide 41. The calcar miller guide has a guidetube 49 attached to the outrigger portion 47. Calcar miller guide 45 isslidably adjustable along guide sleeve 33 in the through hole 39 toproperly position it with respect to the femoral neck seat of thefemoral neck N. The calcar miller guide 45 is fixedly held from rotationwith respect to the guide sleeve 33. The position of the calcar millerguide tube 49 over the femoral neck N defines the axis AX-1.

A number of calcar millers (not shown) are provided having progressivelylarger diameters to gradually increase the side of the hole formed inthe femur F. A final calcar miller 51 (FIGS. 11A-11C) is sized to millthe first bore 21 in the medial endosteum to provide a close fit betweenthe prosthesis 1 and the medial endosteum.

A cannulated pin guide 53 (FIGS. 12A-12D) is sized to be receivedthrough the guide tube 49 of the calcar miller guide 45 and to beslidably received in the first bore B1 created by a calcar miller 51 inthe femoral neck N. The cannulated pin guide 53 has a central axialpassage 55 to slidably receive a trocar point guide pin 57 (FIG. 4N) anda drill point guide pin 59 (FIG. 4P). The trocar point guide pin 57 andthe drill point guide pin 59 have the same diameter, e.g., 3.5 mm.

A cannulated cortex drill 61 (FIGS. 13A-13D) is sized to be slidablyreceived over the drill point guide pin 59. The cannulated cortex drill61 is sized to drill a bore through the posterolateral femoral cortex Cthat is slightly smaller in diameter than the distal stem of theprosthesis 1 (e.g., 9 mm for a 9.5 mm diameter prosthesis stem).

An offset reaming guide, generally indicated at 63 (FIGS. 14A-14E), issized to be slidably received in the first bore B1. The offset reamingguide 63 comprises a trunnion 65 and guide finger 67 mounted on aplatform 69, and a distal end section 71. The distal end section 71 ofthe offset reaming guide 63 is sized (14 mm in the illustratedembodiment) to allow passage through the first bore B1 and has a bulletdistal end to facilitate passage through the first bore. The exteriorshape of the platform 69 (as seen from the ends of the reaming guide 63)is generally that of non-overlapping surfaces of two axially parallel,radially overlapping cylinders (see FIGS. 14C and 14E). A largercylinder 69A of the overlapping cylinders coaxial with the axis of thedistal end section 71 of the reaming guide 63 is larger than that of asmaller cylinder 69B. The smaller cylinder 69B is cut on a plane anglingdownwardly toward the intersection with the larger cylinder 69A. Thelargest transverse dimension of the platform 69 is about 15 mm toprovide line-to-line fit with the first bore B1. However, it is to beunderstood that the transverse dimension of the platform will varydepending upon the size of the bone.

The guide finger 67 is disposed parallel to and generally inregistration with the trunnion 65. The guide finger 67 engages theendosteal wall in the femur F to facilitate holding the trunnion 65 inposition as a cannulated reamer 73 (see FIGS. 15A-15D) cuts the bone.The trunnion 65 is cylindrical and offset about 6 mm from the centrallongitudinal axis of the distal end section 71 of the offset reamingguide 63. The precise offset distance will vary depending upon the sizeof the bone in which the prosthesis 1 will be installed. The trunnion 65is sized to receive the cannulated reamer 73 thereon and to permitrotation of the cannulated reamer on the trunnion for reaming the bonewhile guiding the reamer along a line parallel to the axis AX-1 of thefirst bore B1 formed in the femur F. The cannulated reamer 73 forms thesecond bore B2.

A calcar planing guide, generally indicated at 75, comprises a stem 77including an upper portion 77A and a lower portion 77B, and a trunnion79 generally coaxial with the stem (FIGS. 16A-16D). The calcar planingguide 75 has a bullet shaped distal tip to aid in passage through thefirst bore B1. The shape of the stem 77 is generally the same as that ofthe prosthesis 1 except that the lower stem portion 77B is smooth (i.e.,lacking the splines 19 of the prosthesis). The upper portion 77A of thestem is received in a double bore (first B1 and second B2) arrangementformed in the femur neck. The calcar planing guide 75 fits snugly in thefirst and second bores B1, B2 to hold the planing guide from movingwithin the femur F. The exterior surface of the stem 77 is smooth in theembodiment illustrated in FIGS. 16A and 16B.

However, the calcar planing guide should preferably correspond closelyto the shape of the prosthesis 1. FIG. 16D illustrates an embodiment ofa calcar planing guide 75′ in which the stem 77′ has solines 77C′corresponding identically to the splines 19 of the prosthesis. In theevent the prosthesis 1 also had splines (not shown) on the upper portion15 of its stem 13, similar splines (not shown) would be formed on theupper portion 77A′ of the planer guide stem 77′. By more preciselymatching the shapes of the stems (13, 77′) of the prosthesis 1 andplaning guide 75′, a greater congruency of the underside (9A, 11A) ofthe collar 7 and the seat formed on the neck N may be achieved.

A calcar planer of the present invention (generally indicated at 81)forms a seat for the collar 7 of the prosthesis 1 on the resected neck Nof the femur F (see FIGS. 17A-17C). The calcar planer 81 comprises ahead, generally indicated at 83, and a shaft 85 extending axially fromthe head. The calcar planer head has a central axial passage 87 whichreceives the trunnion 79 of the planing guide 75 therein to mount theplaner on the planing guide for rotation relative to the planing guideon the trunnion. The bottom 89 of the head 83 has the shape of a frustumof a cone. The angle of the cone to a plane perpendicular to the centrallongitudinal axis AX-1 is about 100 when the planer 81 is mounted on theplaner guide 75. The side 91 of the head 83 is also conical in shape,making an angle of about 60° with the plane perpendicular to the centrallongitudinal axis AX-1. The shape of the head 83 corresponds closely tothe shape of the underside (9A, 11A) of the collar 7.

In the event the prosthesis 1″ having a flat underside 9A″ is to beinstalled, the bottom 89′ of the head 83′ of the calcar planer 81′ isalso flat. The calcar planer 81′ having a flat bottom 89′ is illustratedin FIG. 18M. It is believed the use of the flat bottomed calcar planer81′ and prosthesis 1″ increases the chance of obtaining a very highlevel of congruency between the prosthesis and the seat on the neck Nformed by the calcar planer.

(c) Method of Implanting the Prosthesis

The method of the present invention for implanting the prosthesis 1assures close replication of normal loading of the femur F (i.e.,loading prior to implantation of the prosthesis). One preferred methodof the present invention is illustrated in FIGS. 4A, 4B and 4M-4S. Alesser preferred method is illustrated in FIGS. 4A-4L. A most preferredmethod is illustrated in FIGS. 18A-18P. A femoral head-neck prosthesiswhich fails to replicate normal loading conditions will change thestress distribution through the femur F. As mentioned in U.S. Pat. No.4,998,937, incorporated herein by reference, according to Wolff's lawthese changes in stress distribution eventually cause alterations in theinternal structure of the bone. Those portions subject to a lesserstress than before are likely to deteriorate and those subject togreater stress than before are likely to thicken. Excessive increases instress over those associated with normal loading may kill the bone cellsif the stress is applied over an extended period of time. To replicatenormal loading, the method of the present invention aligns the stem 13of the prosthesis 1 with the average compression loading vector for theparticular femur, which vector is variable from person to person.

Referring to FIG. 1A, the human femur F has two externally visible axes:the axis of the femoral neck AX-4 and the axis of the femoral shaftAX-3. However, the bone is not loaded along either of these two visibleaxes, but rather is loaded through a third axis (parallel to the averagecompression loading vector) which is not externally apparent. Inresponse to compressive loading and the strain energy densityexperienced by the femur F, reinforcing lines of bone, which are calledcompression trabeculae, form within the femur. The collection of thesereinforcing lines is the compression trabecular stream TS. Theparticular collection of compression trabeculae in the femur neck, asshown in FIG. 1A, is referred to as the medial trabecular stream TS, andthe average direction of the medial trabecular stream may be referred toas the medial trabecular stream axis AX-5. Angle θ which axis AX-5 makeswith the central longitudinal axis of the femur shaft AX-3 generallyranges from 140 to 170 degrees. In practice, this angle is measured froma profile X-ray of the hip between the axis AX-5 and a lateral surfaceof the femur F (see FIG. 4A). The use of the medial trabecular stream TSto position the prosthesis 1 is discussed in U.S. Pat. No. 4,998,937.

To install the prosthesis 1 in the femur F in accordance with the methodof this invention, the hip joint and the lateral side of the femur arefirst surgically exposed. A vertical plane P-1 through the centrallongitudinal axis AX-2 of the femoral neck is typically at an angle ofapproximately 15 degrees anterior to a lateral-medial plane P-2 throughthe central longitudinal axis AX-3 of the femoral shaft, as shown inFIG. 1B. This angle is commonly referred to as the “anteversion” of thefemoral neck. Accordingly, the angle guide 29 is positioned radially onthe femur F such that the vertical axis of bracket 31 lies in plane P-1approximately 15 degrees posterior from the lateral-medial plane P-2(since the bracket is lateral of axis AX-3 and the femoral neck 7 ismedial). In this position, a vertical plane P-3 through guide sleeve 33should be parallel to plane P-1. In addition, the angle guide 29 ispositioned proximally-distally on the femur F such that the upper end ofguide sleeve 33 is centered with respect to the base of the femoralneck, as shown in FIG. 1B. The angle guide 29 is then clamped on thefemoral shaft S by a clamp (not shown). The guide sleeve 33 is adjusted,by loosening screw 35, relative to the bracket 31 so that its anglerelative to the central axis AX-3 of the femur shaft S matches the anglee of the medial trabecular stream TS. This is accomplished by aligningpointer 37 of the guide sleeve with the appropriate angle indicated onthe faceplate 31C of the bracket 31.

The saw guide 41 is positioned (proximally-distally) on guide sleeve 33such that the sawcut slot 41A is located adjacent the base of thefemoral neck N and generally aligned with the upper surface of thelateral femoral cortex of the femur F, as shown in FIG. 4B. In thisposition, the sawcut slot should be perpendicular to the medialtrabecular stream TS. Set screw 43 is tightened to firmly attach the sawguide 41 in the guide sleeve 33 of the angle guide 29.

With the saw guide 41 in place, the femoral neck N is cut with anoscillating saw (not shown) by passing the saw through the sawcut slot41A to form a cut surface extending from the lateral femoral cortex atan angle of approximately 60 degrees with respect to the centrallongitudinal axis AX-3 of the femoral shaft S. The saw guide 41 is thenremoved from the guide sleeve 33, leaving the angle guide 29 attached tothe femoral shaft S in its original position, and the femoral head H isremoved.

If a total hip replacement (i.e., replacement of the femoral head H andacetabulum (not shown) is required, the acetabulum should now beprepared.

In the first preferred embodiment, as shown in FIGS. 4A, 4B and 4M-4S,the calcar miller guide 45 is secured to the guide sleeve 33, whicheffectively centers the calcar miller guide with respect to the cutsurface of the femoral neck N. The angle guide 29 also aligns the calcarmiller guide 45 parallel to the axis AX-5. A starter hole is drilledinto the femoral neck 7.

A miller (not shown) of relatively small milling diameter is slidablyreceived in he calcar miller guide 45 to mill the femoral neck. Thefemoral neck N is milled by a progression of end and side cuttingmillers, with each succeeding miller having a larger diameter than thepreceding miller. The femoral neck N has an inner lining (or surface)referred to as the endosteum. The final milling diameter is determinedfor the individual femur to provide an appropriate diameter of the firstbore B1 adjacent to the medial endosteum. The calcar miller 51 of theappropriate diameter mills a bore in the medial endosteum to the finaldiameter (e.g., 15 mm). The calcar miller 51 is then removed from thecalcar miller guide 45 (FIG. 4M).

As shown in FIG. 4N, the cannulated pin guide 53 is received in thecalcar miller guide 51 and into the first bore B1 in the medialendosteum. The trocar point guide pin 57 is received in the cannulatedpin guide axial passage 55 to make a starter mark on the lateralendosteum. After the starter mark is made, the trocar point guide pin 57is removed from the passage 55. The drill point guide pin 59 is thenreceived in the cannulated pin guide axial passage 55 and is used todrill through the posterolateral femoral cortex C, forming an obliquehole in the posterolateral femoral cortex. The drill point guide pin 59is left in place after drilling the oblique hole in the cortex, thecannulated pin guide 53 is removed from the femur F and the calcarmiller guide 45 is removed from the guide sleeve 33 of the angle guide29.

As shown in FIG. 4P, the cannulated cortex drill 61 is received over thedrill point guide Dpn 59 to drill through the posterolateral femoralcortex C on the same axis as the first bore B1 milled by the calcarmiller 51. Cortex drill 61 drills the oblique hole to a diameter that issmaller than the first bore B1 formed in the femur F. The cannulatedcortex drill 61 and the drill point guide pin 59 are then removed fromthe femur 3.

The offset reaming guide 63 is then placed into the first bore B1 bulletend first. A first cannulated reamer (not shown) is received on thetrunnion 65 to ream the second bore B2 in the femoral neck N which isparallel to the first bore B1. A progression of cannulated reamers (notshown) are used, with each succeeding reamer having a larger diameter.The final cannulated reamer 73 reams the second bore B2 to a diameterwhich achieves line-to-line contact between the prosthesis 1 and theendosteum (FIG. 4Q). After the second bore B2 is reamed to a depth toaccommodate the upper stem portion 15 of the prosthesis 1, thecannulated reamer 73 and the offset reaming guide 63 are removed fromthe femur F.

Referring to FIG. 4R, the calcar planing guide 75 is inserted into theproximal side of the first bore B1 and through the oblique hole in theposterolateral cortex C. The central longitudinal axis of the trunnion79 and the stem 77 of the calcar planing guide 75 are collinear with thefirst bore B1. The calcar planer 81 is then placed on the trunnion 79and the surface of the femoral neck is planed generally perpendicular tothe axis (AX-1) of the first bore S1 while even pressure is applied tothe calcar planer to form a seat for the collar 7 of the prosthesis 1.The greater trochanter T is substantially preserved by the calcar planer81. Only an angled segment of the trochanter T is cut away providing anangled seat for the flange 11 of the collar 7. In this way, a secureengagement of the prosthesis 1 on the cortical bone C of the upper femuris achieved without sacrificing a substantial portion of the trochanterT (FIG. 1).

The bottom 89 of the calcar planer 81 is slightly cupped so that theportion of the seat on the femoral neck N slopes downwardly toward theaxis AX-1. The shape of this portion of the seat is complimentary tothat of the underside 9A of the prosthesis collar 7. The cup shape ofthe seat on the femoral neck N helps to locate the prosthesis 1.Moreover, when the underside of the collar 7 and the seat are congruent,the entire area of the seat engages the underside (9A, 11A) of thecollar 7 and is subjected to loading by the prosthesis 1. Loading of thebone material of the seat over the entire area of engagement with thecollar surface (9A, 11A) prevents resorption (withdrawing) of the boneafter the prosthesis 1 is implanted. However, although macroscopiccongruence is important, microscopic roughness or porosity of the collarundersurface (9A, 11A) possibly combined with bioactive or chemicalcoating (e.g., calcium phosphate compound) allows an ingrowth of bonefrom the seat which facilitates bonding of the collar surface with theseat. Because the collar undersurface (9A, 11A) achieves one hundredpercent cortical contact and transmits substantially one hundred percentof the cortical loading, the chemical coating is used only on theunderside of the collar 7 and at no locations on the stem 13. Afterplaning, the calcar planer 81 and the calcar planer guide 75 areremoved.

The prosthesis 1 (without the ball 3) is then implanted by driving thestem 13 into the first bore B1 as shown in FIG. 4S. The splines 19 ofthe stem bite into the walls of the first bore B1 and the stem protrudesslightly through the oblique hole so that cortical bone does not latergrow over the end of the stem. Growth of bone over the end of the stem13 would be undesirable since it would impede the ability of theprosthesis 1 to transmit loads from the hip to the upper femur. Theupper stem portion 15 fits closely into the first bore B1. The underside9A of the collar platform 9 is congruent with the portion of the seatwhich was formed by the bottom 89 of the calcar planer head 83 and theunderside 11A of the flange 11 is congruent with the portion of the seaton the trochanter T formed by the side 91 of the calcar planing head.

Once the prosthesis 1 is implanted, an appropriately sized ball 3 isthen locked onto the neck.

In a second, lesser preferred embodiment, the procedure is somewhatmodified. Referring to FIGS. 4C and 4D, after the femoral neck N isresected, a reamer guide 95 (similar in construction to the calcarmiller guide 45) is secured to the guide sleeve 33 of the angle guide29, which effectively centers the reamer guide over the femoral neck Nso that the second bore B2 is formed first. As before, the second boreB2 is formed by a progression of reamers (not shown), with eachsucceeding reamer having a larger diameter than the preceding reamer.The final reamer 97 has a diameter of 21 mm, so that the femoral neck Nis reamed to a diameter of 21 mm. The reamer guide 95 is then removedfrom the guide sleeve 33, leaving the angle guide 29 attached.

A calcar miller guide 99 having a trunnion 101 is attached to the angleguide 29 and the first bore B1 is formed by milling with a series ofcalcar millers including final calcar miller 103 (FIGS. 4E and 4F). Theangle guide 29 is removed in FIG. 4F to more clearly show the calcarmiller guide 99. The calcar miller guide 99 is removed from the angleguide 29 and a drill pin guide 105 is mounted on the angle guide.Referring to FIG. 4G, the upper portion of the drill pin guide 105 has adouble cylinder construction similar to the upper portion 15 of theprothesis stem 13 to fit in the first and second bores, 21 and B2. Theangle guide 29 is also not shown in FIG. 4G. The same trocar pin 57 anddrill pin 59 in the more preferred embodiment are used with the drillpin guide 105 of the lesser preferred embodiment to start the distalhole in the posterolateral cortex of the femur F. The planing step andinstrumentation are substantially the same as described for the methodof the first more preferred embodiment.

In a third, most preferred embodiment shown in FIGS. 18A-18P, theprocedure and some of the tools are modified. The initial step (FIG.18A) of determining the angle of the medial trabecular stream TS iscarried out exactly as described above in reference to FIG. 4A. Once theangle of the medial trabecular stream with respect to the centrallongitudinal axis AX-3 of the bone is determined, the angle of the guidesleeve 33 of the angle guide 29 is set as described above. In the mostpreferred embodiment, this angle is checked using a protractor deviceindicated generally at 101. The protractor device has a stop 103 againstwhich the bracket 31 of the angle guide 29 is placed. A pivotable arm105 can be moved, by loosening set screw 107, to the angle correspondingto the angle of the medial trabecular stream TS. The arm 105 is fixed bythe screw 107 and the guide sleeve 33 should be in face-to-faceengagement with the arm. If not, the sleeve guide 33 is turned until itmatches the angle of the arm 105 of the protractor device 101. Thelarger scale of the protractor device 101 permits a more accuratesetting of the angle guide 29.

As illustrated in FIG. 18C, the angle guide 29 is attached to thesurgically exposed femur and the saw guide 41 is secured in the angleguide. The procedure for resecting the femoral head is the same asdescribed above in reference to FIG. 4B. The saw guide 41 is removedfrom the angle bracket and replaced with a visual sighting bar 109 whichextends generally upwardly from the guide sleeve 33 and beyond theresected neck N of the femur. As shown in FIGS. 18D and 18E, a reamer111 is then directed by the surgeon along the angle indicated by thevisual sighting bar 109 into the femur to form an initial hole HO in thefemur. The hole HO thus formed has a longitudinal axis parallel to themedial trabecular stream TS and with the proper anteversion, both ofwhich are indicated by the bar 109. The reaming may be carried out usinga reamer 111 and a number of succeeding reamers of increasingly largerdiameter to form the full diameter of the hole. A skilled surgeon canalternatively form the full diameter of the hole HO using only thereamer 111. In that event, the surgeon moves the reamer 11 inprogressively larger circles until the full diameter is reached. Thefinal diameter of the hole HO is dictated by the size and shape of thefemur of the individual patient.

Sizing of the hole for the best fitting prosthesis is carried out usingproximal femoral sizers 113 such as the sizer shown in FIG. 18F. Thesizer 113 is substantially identical in shape to the upper portion 15″of the stem 13″ of the prosthesis 1″ to be implanted. The sizers rangein size. For example the larger cylindrical portion 113A of the sizermay range in size from 18 to 26 mm in one millimeter increments. Thesizers 113 are inserted into the hole HO parallel to the visual sightingbar 109 (not shown in FIG. 18F). Progressively larger sizers are fittedinto the proximal femur to determine the dimensions of the largestprosthesis which will fit in the femur. The surgeon now knows the sizeof the prosthesis 1″ to be implanted and the exact dimension of thebores B1, B2 needed to receive the prosthesis. The prosthesis 1″ isselected to be a size larger than the largest sizer 113 which is able tofit in the hole HO. For example, if the largest sizer that would fit hada smaller diameter portion of 14.5 mm a prosthesis having a smallerdiameter upper stem portion of 15 mm would be used.

A reamer 115 having a diameter corresponding to the final diameter(e.g., 15 mm) of the first bore B1 is selected and used to form thefirst bore. As shown in FIG. 18G, the reamer is guided freehand usingthe visual sighting bar 109. The first bore BE is also formed so that atleast a portion of the bore is defined by the endosteum of the medialfemoral cortex. Thus, the prosthesis 1″ when installed will engage thehard cortical bone at this location in the bore B1. In order to drill ahole in the posterolateral femoral cortex which is precisely parallel tothe medial trabecular stream TS and with the proper anteversion, aseries of one piece, cannulated outrigger pin guides 117 (only one isshown) of different sizes are provided. The pin guide 117 having adiameter corresponding to that of the newly formed bore B1 is selectedand inserted into the bore. The outrigger portion of the pin guide 117is received in the guide sleeve 33 of the angle guide 29 at the sametime it is inserted into the bore B1 for the most precise alignment ofthe pin guide. The tapered tip of the guide is advanced into the firstbore B1 until it makes contact with the endosteum of the lateral femoralcortex.

A drill guide pin 119 is inserted into the pin guide 117. Theorientation of the guide pin 119 in the femur is checked by making surethe angle guide 29 is still at the proper angle parallel to the medialtrabecular stream TS and with the proper anteversion. Moreover, the pinguide 117 is checked to make sure it is in contact with the endostealsurface of the medial neck cortex. A drill (not shown) is attached tothe guide pin 119, and it is drilled through the lateral femoral cortex.As shown in FIG. 18I, the pin guide 117 and the angle bracket 29 areremoved from the femur, leaving on the guide pin 119. The next step isto drill the transcortical tunnel through the posterolateral femoralcortex. A cannulated drill 121 is selected which corresponds to thediameter of the lower portion 17″ of the prosthesis 1″. For example ifthe diameter of the lower portion 17″ is to be 9.5 mm, a 9 mm cannulateddrill is selected. As illustrated in FIG. 18J, the cannulated drill 121comes with a guide ferrule 123 which is sized according to the diameterof the bore B1. Thus as shown in FIG. 18K, the guide ferrule 123 helps(along with the guide pin 119) to align the drill with the longitudinalaxis of the bore B1. The cannulated drill 121 slides over the guide pin119 with the guide ferrule 123 received on the drill and thetranscortical tunnel is formed in the posterolateral femoral cortex.Care is taken during drilling to make sure the ferrule 123 remainsagainst the medial endosteal cortex.

The second bore B2 is formed as shown in FIG. 18L, which is the sameprocedure as described above in relation to FIG. 4Q. However, an offsetreaming guide 63′ is shown in FIG. 18L which has splines 71A′ for moreprecise orientation of the reaming guide in the femur.

As shown in FIG. 18M, the calcar planing guide 75′ inserted into thebores B1, B2 is virtually identical in shape to the prosthesis 1″. Theplaning guide 75′ has splines 77C′ on its lower stem portion 77B′ likethose of the prosthesis. The calcar planer 81′ is received on thetrunnion 79′ of the planing guide 75′ and a flat seat is formed on thefemoral neck N. There is preferably a very close tolerance between thetrunnion 79′ and the planer 81′ to avoid wobble as the planer is rotatedto form the seat.

Referring to FIG. 18N the prosthesis 1″ is inserted into the femur usinga guide tip 125 which is attached to the end of the prosthesis. Moreparticularly, the guide tip 125 has a stem (not shown) which is receivedin a hole (not shown) in the distal end of the prosthesis 1″. The tip125 is held by a friction fit in the hole. The bullet nosed shape of thetip 125 helps to keep the prosthesis from hanging up on the bone beforeit passes through the posterolateral femoral cortex. Once implanted, thetip 125 can be pulled off of the prosthesis l″ as shown in FIG. 18P. Theprosthesis 1″ is checked to make certain that the collar 7″ is fullyseated on the seat of the femur neck N, and checked for the appropriateamount of stem protrusion from the femur.

FIGS. 19A-19D illustrate an additional step which may be performed tomake absolutely certain that the a collar 7′″ of a prosthesis 1′″ ofstill another embodiment has seated fully against the femoral neck N.The prosthesis 1′″ differs from the prosthesis 1″ only in that theunderside 11A′″ of its flange 11′″ is composed of two intersectingplanes. The underside 11A″ of the prothesis flange 11″ has the shape ofa conical section. A saw template, generally indicated at 127, includesa cap 128 capable of fitting on the neck 5′″ of the prosthesis 1′″. Thecap 128 precisely locates slots 29 just under the collar 7′″ and to themedial side of the flange 11′″.

A saw (e.g., oscillating saw SW) may then be used to cut away additionalportions of the femoral neck N under the collar 7′″. The saw template127 guides the saw SW and a reciprocating saw (not shown) for makingcuts which are closely congruent with the shape of the underside (9A′″,11A′″) of the collar 7′″. The planer shape of the underside 11A′″permits linear cuts to be made (e.g., as by the blade B′ of thereciprocating saw in FIG. 19D) adjacent to the flange while achieving ahigh degree of congruency between the cut surface and the flangeunderside. After removal of these portions, the underside of the collar7″ is irrigated of remaining debris and the area is checked forcompleteness of the removal. The prosthesis 1′″ is then drivendownwardly (e.g., 1 or 2 mm) after the portions are removed for a morecongruent seating against the neck N.

The slots 129 are disposed in a rim 131 of the saw template 127.Referring to FIG. 19D, the rim 131 has a peripheral edge 133 which isshaped so that the distance of the edge to the upper portion 15′″ of thestem 13′″ of the prosthesis 1′″ is everywhere constant. The peripheraledge 133 is engaged by stop bolts SB on blades B of the saw SW to limitthe inward travel of the saw blade. Thus, the shape of the rim 131assures that the blades B of the saw SW will not contact the upperportion 15′″ of the stem 13′″.

It is envisioned that the saw template 127 could be used in place of theplaning guide (75, 75′) and calcar planer (81, 81′). The other steps forimplantation of the prosthesis 1′″ would be the same as shown indescribed above for the lesser preferred, more preferred and mostpreferred methods. However, there is no planing of the neck N after thefemoral head H is resected to form a seat for the collar 7′″. Theprosthesis 1′″ is driven into the femur F until the underside (9A′″,11A′″) of the collar 7′″ engages the neck. The saw template 127 isattached to the prosthesis 1′″ and the bone is cut under the collar 7′″to form the seat for the collar. The saw template 127 may also be usedbeneficially in the removal of a previously implanted prosthesis 1′″.Bone ingrowth into the prosthesis 1′″ is promoted (as described above)only on the underside (9A′″, 11A′″) of the collar 7′″. Minimal amountsof bone would be removed using the saw template 127 to separate theunderside (9A′″, 11A′″) of the collar 7′″ from the femur F.

(d) Study Regarding Present Invention

In total hip arthroplasty (THA), intramedullary stem femoral componentsdecrease strain levels in the proximal femur resulting in periprostheticbone loss. This study evaluates the strain pattern of the femoral stemdesign of the present invention in comparison to a normal femur and toconventional femur head-neck prostheses.

Attempts to eliminate strain deprivation bone loss in THA by means ofreduced stiffness intramedullary stems have been unsuccessful(1). Ahuman study of an instrumented femoral prosthesis found the loadtrajectory of the hip to fall within a relatively narrow range ofangles(2). An alternative approach to improve proximal femoral loadingis to align the femoral stem parallel to the average resultant loadingvector of the individual hip. In theory, unimpeded loading of thefemoral neck through a stable interface should generate strain levelsequivalent to the intact femur. To enable unrestricted collar-neckloading, it is necessary for the implant stem to go through the bone inline with the resultant vector. The purpose of this study was todetermine the strain distribution of a transosseous THA prosthesis.

Twelve synthetic femurs were bonded with twelve triaxial rosette straingages (e.g., strain gages 109 shown in FIG. 1), five each along theposteromedial and anterolateral aspect and one each proximal-anteriorand proximal posterior. The femurs were mounted in a single limb stancejig. Spinal loads of 1068 and 2135 Newtons were applied with simulatedabductor force of 712 and 1423 Newtons creating a resultant 21° from thefemoral shaft axis. Strain data were acquired with a computerizedmulti-channel system which converted the readings to microstrain.

Prototype cobalt-chrome transosseous femoral stems constructed accordingto the principles of the present invention were installed and loadedunder the same conditions as the intact femurs. The collar was 10°conical and perpendicular to the stem. The proximal stem consisted oftwo cylindrical elements 23, 25 of 15 and 21 mm diameter, respectively,and achieved tangential contact with the endosteal cortex. The distalstem, which was fluted and 12 mm in diameter, was press-fit through an11.5 mm hole in the posterolateral cortex. Two distal stem variationswere tested in each femur: slotted (n=11) and solid (n=12). Radiographsof each femur were obtained. The angle of implantation varied from 1460to 1580 in relation to the lateral shaft cortex.

Eight non-cemented and cemented cobalt-chrome intramedullary stems(Replica, 16.5-LG and Response, 13.5, manufactured by DePuy, Inc. ofWarsaw, Ind.) were installed and tested.

Analysis of variance was performed on all data.

Comparable strain patterns were noted at each of the two loadingconditions. The following results are from the higher load condition. Agraphic representation of the results appears as FIG. 18.

The non-cemented and cemented intramedullary stems resulted in proximalposteromedial compression strain levels of 42.7±4.6% (mean±SEM,p=0.0007) and 32.3±2.6% (p=0.0001) compared with the intact condition.

The slotted and solid transosseous stems produced proximal posteromedialcompression strain levels of 119.0±7.4% (p=0.36) and 101.1±16.6%(p=0.66). Compression, tension and shear strain levels were generallynot significantly different from intact levels. Exceptions includedincreased tension strain at the most proximal posteromedial gage withthe slotted stem, 128.9±2.7% (p=0.029). Significantly increasedcompression strain was noted at the gage nearest the stem exit site withthe slotted and solid stems (146.1±11.2%, p=0.0096 and 188.6±11.9%,p=0.0006). A trend was noted of higher proximal strain levels with amore horizontal angle of implantation, however this was not significanton linear regression analysis.

The diminished strain levels noted with the intramedullary stem femoralcomponents were consistent with those previously reported(3). Althoughsignificant effort has been expended attempting to resolve the “modulusmismatch” of intramedullary stems, the results of this study suggestthat a trajectory mismatch may be a more significant factor in strainreduction. The compression trabeculae of the hip were found to be 10 to40° more horizontal than the axis of the femoral shaft(4). Theconsequent bending moment on intramedullary stem components impedesproximal interface loading. The loading trajectory of the hip is morehorizontal than intramedullary stem insertion trajectory (femoral shaftaxis) which creates a bending moment.

A trajectory matched femoral component (stem aligned with loadingvector) incurs a smaller bending moment and receives a more axial loadtransmission. The cylindrical machining and corresponding stem of thistransosseous design seeks to resist rotation and toggle with proximaland distal cortical contact/macrointerlock, but accommodates collar-neckinterface compression. Although proximal femoral strain levels wererestored with this prototype, high strain levels near the distal stemwould raise concern for potential thigh pain.

Within a synthetic femur strain model, transosseous THA femoralcomponents demonstrated higher proximal femoral strain levels thanintramedullary stems.

REFERENCES

(1) Bobyn, J. D. et al.: CORR 261:196, 1990

(2) Davy, D. T. et al.: JBJS 70A:45, 1988

(3) Oh, I. and Harris, W. H.: JBJS 60A:75, 1978

(4) Clark, J. M. et al.: J Arthr 2:99, 1987

In view of the above, it will be seen that the several objects of theinvention are achieved and other advantageous results attained.

As various changes could be made in the above constructions withoutdeparting from the scope of the invention, it is intended that allmatter contained in the above description or shown in the accompanyingdrawings shall be interpreted as illustrative and not in a limitingsense.

What is claimed is:
 1. A femoral prosthesis for implantation in a femur,the prosthesis comprising a neck adapted to receive a ball thereon, acollar on which the neck is mounted and a stem extending from the collaron the opposite side of the collar from the neck, the prosthesis havinga longitudinal axis corresponding to the longitudinal axis of the stemand being configured for transosseous implantation in which the stementers the bone generally at one side thereof, crosses a medullary canaland enters cortical bone on an opposite side of the bone, the stem beingconstructed and arranged to fix the prosthesis from movement about itslongitudinal axis and about axes perpendicular to the longitudinal axisand to inhibit axial fixation of the stem upon implantation in thefemur, thereby to achieve substantially natural loading of the upperfemur.
 2. A femoral prosthesis as set forth in claim 1 wherein the stemcomprises outwardly facing fixation surfaces for fixing the prosthesisto the femur, the fixation surfaces being generally parallel to thelongitudinal axis of the prosthesis.
 3. A femoral prosthesis as setforth in claim 2 wherein an upper stem portion has the shape of twoparallel, partially radially overlapping cylinders.
 4. A femoralprosthesis as set forth in claim 2 wherein the stem has axiallyextending splines formed thereon for fixing the prosthesis from movementabout its longitudinal axis and about axes perpendicular to thelongitudinal axis and to inhibit axial fixation of the stem uponimplantation in a femur.
 5. A femoral prosthesis as set forth in claim 2wherein the stem includes an upper portion, fixation surfaces of theupper portion being substantially smooth to inhibit bone ingrowth intothe fixation surfaces of the upper portion.
 6. A femoral prosthesis asset forth in claim 5 wherein the stem has axially extending splinesformed thereon for fixing the prosthesis from movement about itslongitudinal axis and about axes perpendicular to the longitudinal axisand formed to inhibit axial fixation of the prosthesis upon implantationin a femur.
 7. A femoral prosthesis as set forth in claim 1 wherein thecollar has an underside constructed and arranged for engaging the upperfemur for transmitting loads to the upper femur, the collar comprising afirst portion and a second portion, an underside of the second portionengageable with the femur being oriented at an angle to an underside ofthe first portion.
 8. A femoral prosthesis as set forth in claim 7wherein the first portion of the collar comprises a neck platform onwhich the neck is mounted, and the second portion of the collarcomprises a flange disposed for engaging the greater trochanter of thefemur.
 9. A femoral prosthesis as set forth in claim 7 wherein theundersides of the first and second portions of the collar are of aporous construction to encourage bone ingrowth into the collar, theremainder of the prosthesis being free of porous construction.
 10. Afemoral prosthesis as set forth in claim 1 wherein the collar is sizedand shaped to cap the medullary canal of the femur exposed afterresection of the femoral head thereby to prevent migration of debrisinto the medullary canal.
 11. A femoral prosthesis as set forth in claim1 wherein the neck has a longitudinal axis parallel to the longitudinalaxis of the stem.
 12. A femoral prosthesis as set forth in claim 1 incombination with the ball.
 13. A femoral prosthesis as set forth inclaim 1 in combination with a calcar planing guide adapted for use inplaning a surface of the femur to form a seat, the planing guideincluding a stem sized and shaped substantially identically to the stemof the prosthesis stem thereby to facilitate precise planing of thefemur surface to form the seat, an upper portion of the planing guidestem having the shape of two parallel, partially radially overlappingcylinders.
 14. A femoral prosthesis as set forth in claim 1 incombination with a saw template constructed for mounting on the neck ofthe prosthesis, the saw template having a saw guide slot positioned whenmounted on the neck for precise sawing of the femur under the prosthesiscollar to remove bone and facilitate a congruent fitting of the collaron the femur.
 15. A femoral prosthesis as set forth in claim 1 whereinthe stem has axially extending splines formed thereon for engaging thefemoral bone and fixing the prosthesis from movement about itslongitudinal axis and about axes perpendicular to the longitudinal axisand to inhibit axial fixation of the stem upon implantation in a femur.16. A femoral prosthesis as set forth in claim 1 wherein the neck,collar and stem are integrally made of one piece of material.
 17. Afemoral prosthesis for implantation in a femur, the prosthesiscomprising a neck adapted to receive a ball thereon, a collar on whichthe neck is mounted and a stem extending from the collar on the oppositeside of the collar from the neck, the prosthesis having a longitudinalaxis corresponding to the longitudinal axis of the stem, the neck havinga longitudinal axis parallel to the longitudinal axis of the stem, thecollar being continuous about the circumference of the prosthesis,extending outwardly laterally, anteriorly, medially and posteriorly andsized and shaped to cap the medullary canal of the femur exposed afterresection of the femur thereby to prevent migration of joint wear debrisinto the medullary canal.
 18. A femoral prosthesis as set forth in claim17 wherein the collar has an underside constructed and arranged forengaging the upper femur for transmitting loads to the upper femur, thecollar comprising a first portion and a second portion, an underside ofthe second portion engageable with the femur being oriented at an angleto an underside of the first portion.
 19. A femoral prosthesis as setforth in claim 18 wherein the first portion of the collar comprises aneck platform on which the neck is mounted, and the second portion ofthe collar comprises a flange disposed for engaging the trochanter ofthe femur.
 20. A femoral prosthesis as set forth in claim 18 wherein theundersides of the first and second portions of the collar are of aporous construction to encourage bone ingrowth into the collar, theremainder of the prosthesis being free of porous construction.
 21. Amethod for implanting a non-cemented femoral head-neck prosthesis in afemur, the femur having a shaft and a neck at the upper end of the shaftat the medial side of the femur, the method comprising the steps of:determining the axis of the medial trabecular stream of the femur;cutting the neck of the femur to form a seat on the femur neck; drillinga first bore along a line through the shaft of the femur to extend fromthe neck of the femur down toward the lateral side of the femur along aline substantially parallel to the axis of the medial trabecular stream;drilling a second bore through the shaft of the femur to extend from theneck of the femur down toward the lateral side of the femur along a linesubstantially parallel to the axis of the medial trabecular stream butspaced from the line of the first bore; inserting a stem of theprosthesis in one of the first and second bores extending through theshaft to the lateral side of the femur, a portion of the stem beingreceived in the other of the first and second bores.
 22. A method as setforth in claim 21 further comprising the steps of providing a prosthesishaving a neck with a longitudinal axis parallel to a longitudinal axisof the stem of the prosthesis, and installing the prosthesis in thefirst and second bores so that the longitudinal axes of the stem andneck are parallel to the medial trabecular stream.
 23. A method as setforth in claim 21 wherein the step of inserting the stem comprisesdriving the stem through said one bore until a distal end of the stemprotrudes from the lateral side of the femur.
 24. A method as set forthin claim 21 wherein the first and second bores have generallycylindrical walls and together have a shape in cross section ofpartially radially overlapping cylinders, the portions of the stemcontacting the bore walls being generally cylindrically shaped.
 25. Amethod as set forth in claim 21 further comprising the step, followingthe step of cutting the neck, of milling the neck to shape the seat, themilling cutting a trochanter of the femur at an angle to provide a seatfor the prosthesis while preserving most of the trochanter.
 26. A methodas set forth in claim 21 further comprising the steps of: mounting a sawtemplate on the prosthesis as inserted into the femur; cutting the femurneck under a collar of the prosthesis using the saw template to formsurfaces on the neck corresponding in shape to the underside of thecollar; driving the prosthesis down onto the surfaces formed.
 27. A boneprosthesis for implanting in a bone at a joint, the prosthesiscomprising a neck adapted to receive a ball, a collar on which the neckis mounted and a stem extending from the collar on the opposite side ofthe collar from the neck, the prosthesis having a longitudinal axiscorresponding to the longitudinal axis of the stem and being configuredfor transosseous implantation in which the stem enters the bonegenerally at one side thereof, crosses a medullary canal and enterscortical bone on an opposite side of the bone, the stem beingconstructed and arranged to fix the prosthesis from movement relative tothe bone about its longitudinal axis and about axes perpendicular to thelongitudinal axis and to inhibit axial fixation of the stem uponimplantation in the bone.
 28. A femoral prosthesis for implantation in afemur, the prosthesis comprising a neck adapted to receive a ballthereon, a collar on which the neck is mounted and a stem extending fromthe collar on the opposite side of the collar from the neck, theprosthesis having a longitudinal axis corresponding to the longitudinalaxis of the stem, the neck having a longitudinal axis parallel to thelongitudinal axis of the stem thereby to achieve substantially naturalloading of the upper femur.
 29. A femoral prosthesis for implantation ina femur, the prosthesis comprising a neck adapted to receive a ballthereon, a collar on which the neck is mounted and a stem extending fromthe collar on the opposite side of the collar from the neck, an upperportion of the stem having the shape of two parallel, partially radiallyoverlapping elements.